Electric discharge device



June 13, 1944. D. E. ELMENDORF 2,351,254

ELECTRIC DI SCHARGE DEVICE Filed July 25, 1941 2 Sheets-Sheet 1 Power- Circuit Fig 3.

Heating 35 Circui'i: 30

30 f F 5a Inverfior' Dur'ge E. ELmenclor'f',

His A'tforneg.

June 13, 1944. D. E. ELMENDORF 2,351,254

ELECTRIC DISCHARGE DEVICE Filed July 25, 1941 2 Sheets-Sheet 2 /5 I, /C A\ 3 Heart-m2 i 1 Cir'cui'i: la L Fig. 7. Heating /6 L56 Heat-mg 3/ 3/ Circuif C'n'cui't' lnventov DUT'HGS E. Elmendor'f,

Patented June 13, 1944 ELECTRIC DISCHARGE DEVICE 1 Duryea E. Elmendorf, Cleveland, Ohio, assignor to General Electric Company, a corporation of New York Application July 25, 1941, Serial No. 404,023

5 Claims.

This invention relates to electric discharge devices and systems, and aims at improving and simplifying them in various ways. One advantage that can be obtained through the invention is the elimination of troublesome accessories, including starting and current-limiting means like ballasts, thermal cut-out switches, etc. Another advantage is improvement of the voltage-current characteristics of electric discharge devices, especially discharge devices containing gas or vapor-even to the extent of enabling such devices to start and operate with a positive voltage-current characteristic, and without resort to specially high voltage for starting. The invention is applicable to discharge devices producing useful radiations for various purposes,.such as germicidal tubes or lamps, and lamps used industrially for irradiating or treating many substances and products, and for fluorescent lighting. I have hereinafter explained the invention with particular reference to a device suitable for use as a fluorescent lamp.

In a discharge device to operate with a positive voltage-current characteristic in accordance with my invention, a cathode is provided which displays such a characterictic when operating within a proper normal voltage range,

In a positive column discharge device embodying such a cathode, its positive voltage-current characteristic predominates over the negative voltage-current characteristic of the positive column portion of the discharge-provided, of course, the voltage on the device is not excessive in relation to its length, and that this length is not inordinately great. Whether of positive column type or not, discharge devices with my cathode of positive voltage-current characteristic can be operated without ballast or other current limiting means, and can be started in any way which does not involve operation, after starting, under excessive voltage. One convenient way of accomplishing this is to provide a thermionic cathode which is heated to render it emissive for starting, also using an auxiliary anode if the distance of the operating anode from the cathode is such that a discharge cannot start between the cathode and said operating anode on a voltage within the proper operating range of the device. Naturally, the

device may contain a gaseous or 'vaporousatmosphere during operationconsisting of any suitable ionizable gas or mixture of such gases;=

ing substance or substances-or a mixed atmosphere comprising both gases and vapors.

As suggested above, my invention includes a novel type of electrode or cathode structure for electric discharges in atmospheres of gas or vapor, which exhibits positive voltage-current characteristics when the voltage of the discharge is not excessive. In forms described hereinafter, my novel electrode involves emissive cathode means surrounded by what is in effect a barrier of negative potential for segregating a region adjacent the emissive cathode means and preventing or limiting entry into this region of positive ions from outside the barrier, while nevertheless permitting free passage of the discharge atmosphere of gas or vapor. In some forms, the more active cathode means consists of the interior surface of the barrier structure itself; in other forms, the barrier structure surrounds a separate activated electrode. In these latter forms, these members of the cathode structure (barrier and cathode proper) may coact with one another as electrodes for one (auxiliary) discharge, while either of them may function as cathode for another (main) discharge. In the forms first mentioned, on the other hand, the barrier may serve as cathode for an auxiliary or starting discharge, as well as for the main discharge.

Other features and advantages of the invention beside those already indicated will appear from the following description of species and forms of embodiment, and from the drawings. Indeed, all the features and combinations shown or described are of my invention, so far as' they are novel.

In the drawings,

tric discharge device conveniently embodying my invention; and Fig. 2 is a general view, largely diagrammatic, showing the current connections and the electrode structures of the device, on a larger scale than Fig. 1, one of said electrode structures being in vertical mid-section.

Fig. 3 is a view similar to Fig. 2 showing a modification of one of the electrode structures.

Fig. 4 is a view similar to Figs. 2 and 3 showing other forms of electrode structure; and Fig. 5 is a similar view showing the same electrode structures as Fig. 4 with dili'erent electrical connections.

Fig. 6 is a general view, largely diagrammatic, illustrating a device having at its two ends identical electrode structures like one of those in Fig. 2, with electrical connections for operation on alternating current; and Fig. 7 is a view similar to Fig. 6 showing a device having at its two ends identical electrode structures like one of those in Fig. 4, with electrical connections (diflerent from those in Fig. '6) for operation on alternating current.

The device shown'in Fig. 1 comprises a tubular vitreous envelope l with electrode structures A, C mounted in its opposite ends, and rather widely spaced apart. In the present instance, the electrode structures A, C are dissimilar in function and construction. The right-hand electrode structure A is shown-as a simple anode in the form of a circular metal-ring H attached to the end of a support and current lead l2 sealed through a stem-press -l3 forming this The ring H may be end of the envelope in. v arranged centrally in the tube l0 and perpcndi cular to its axis. The left-hand electrode-struc-.---

ture C is shown as comprising an electrode l5 of'hollow, tubularopenwork or grid-like character, consisting. in the present instance of a helical wire coil, together with, another electrode IS in relativelyclose proximity to theelectrode l5, so as to assist in starting a discharge between the electrodes l5 and A. As' shown in Fig. 2,

the wire coil electrode I5 is athermionic cath-,

ode that .is rendered interiorly emissive '1by. .a coatingof activating material I! on the inner surfaces of its members or'convolutions, and can be heated to a sufilcient temperature to produce. such internal emission of electronsby passage of.

current through the wire. The members or convolutions of the' tubular electrode l5 are rather closely spaced, and refractory flanged insulators l8, it are mounted in its ends, as barrier means to prevent electric discharge from passing freely through theends to the, interior of the electrade, or vice-versa. Thus the electrode structure is closed, and closes oil the region adjacent the emissive cathode surfa ces,against free electric discharge to the interior and the cathode means thre. The electrode I6 is here shown as a wire ringer hoop 'coaxially surrounding the electrode coil [5 nearly at mid-length of the coil, and is attached to a supporting current lead l9 that is sealed 'through a stem-press 20 forming this-end of thee'nvelope H). The ends of the electrode c'oil f5'are connected to supporting leads 2|, 22 th'at are sealed through the stempress 20. As here shown, the lead 22 is bent around to extend axially within the coil l5, through axial holes in the insulators I8, '18; thus helping to hold the electrode'li in shape. so

to speak, besides supporting it eifective ly'b The electrode l5 may be arranged lengthwise of the envelope tube 10, coaxial therewith. 'As here shown, the electrode l6 lies in a transverse plane a little nearer'the anode'Athan mid-length of the electrode l5: however, this electrode'lfi may. be shifted either way along the tube In 'relative to theelectrode 15, to the rightof-thelatten.

The envelope to should contain a low-pressure atmosphere of suitablestarting' as (such for'exmple as argon atan absolute pressure'of about and may very -welt be at present I prefer a gas pressure near the lower figure of mm. above mentioned. In Fig. 1, an internal coating of fluorescent material or phosphor 25 is indicated on the envelope I0.

Fig. 2 shows one suitable system of current supply connections for the device. As here shown, the main power circuit 30 (which may be either 11 C. or A; C.) is connected between the leads H; "it across the electrodes A and I5. A heating power-circuit 3| (D. C. or A. C.) is connected between the leads 2|, 22 to heat the electrode coil l 5.

The voltage in this circuit 3| should, of course, be lower than the ionizing hotentlalis) of the atmosphere in the envelope ID to prevent arcing along the electrode coil. An auxiliary power circuit (D. C. or A. C.) is connected across the electrodes i 5 and l6-being here shown as provided by one side of the circuit 30 and by a circuit lead 32 connected from the other side of the circuit to the lead l9, and including a resistor 33. The voltage applied by this auxiliary'power'cireuit across the electrodes l5, l6 should be relatively low as compared to that applied across the electrodes A and I5 from the; main power circuit, though suflicient (initially, at least) to ionizae the starting gas, as w ll as the vaporizabl working substance. The polarization of the connections is made such that the electrode [5 is negative to the electrode I B-when the anode A. is positive in relation to both of said electrodes. Control switches 35, 36 areshowninterposed in the circuits 3 l. 32. A milliammeter 3'1 and an ammeter 38 are shown I at;, s uitahle points in the auxiliary and main circuits 32- and- 30 for measuring the current values .of the auxiliary and main discharges.

'I'hvese, howevenare of course unessential, and

have no connection with my present invention.

The device being connected and the circuits energized as above described, the cathode I5 is heated and rendered emissive from its inner surface l1, and a-discharge is thus started between the electrodes 15, i6, ionizing the starting gas and, ultimately, the mercury or other vapor that evolves as the envelope l0 heats up. If the electrade 16 is close enoughto the cathode I5, and the voltage between them is sufliciently low, this discharge may havev the character of a negative cathodic glow; otherwise, it may partake of the character of .a positive column discharge. The electrode l6 acts as a starter anode in relation to. the cathode l5,.providing excitation of the starting gas under which the device breaks down .1: to the anode A, so that a positive column arc discharge is. established between the electrodes l5 and A. The'heating of the cathode l5 by this main discharge soon renders heating thereof from the circuit 3| unnecessary, so that the switch 35 in this circuit may be opened. If the to 5 mm. of mercury), and may also contain.

substance (indicated sufiicient in amount device, so that the pressure ofvapor dining-0D- eration will depend-onthe temperature of the' While neitherthevapor pres:-'

envelope .walls. surenor that of the starting gas is critical, yet

power-line 30 is D. C., the starting of the discharge between the electrodes l5 and A also renders unnecessary the continued energization of the auxiliary power circuit between electrodes l5 and 18 that includes the line'32, and its switch 38 may also be opened, but if the power line 30 is A. C., energization of this auxiliary circuit must be continued, to restart the arc after its extinction at each reversal of voltage in the main power circuit 30.

It will readily be apparent to those skilled in the art that the use of electrode l6 for starting,

as above described, may be dispensed with when the electrodes 15 and A are arranged close enough to one another to allow of starting the discharge directly between them, on the normal operating voltage. Or, to state the matter differently, use of the electrode A for normal operation may be done away with when the location of the electrode l6 relative to the electrode I is such as to give an adequate volume of discharge in the device for the purpose in view; and the electrode l6 may be positioned as far from the electrode l5 as will allow of starting the discharge between them on the normal operating voltage of the device.

Under a relatively low voltage suflicient to ionize the initial atmosphere in the envelope l0. and with the auxiliary electrode l6 sufficiently close to the cathode l5e. g., at a distance of the order of the mean free path of the positive ions, more or less-the discharge between the electrodes l5 and I6 takes place under relatively low pressure between relatively large available electrode areas and at relatively low current density, and is largely in the nature of a negative cathodic glow discharge. As the activating material I1 is on the interior of the cathode electrode IS, a high negative space charge of electrons accumulates inside this cathode, which limits the further emission of electrons and the ionization of gas outside the cathode by electrons coming out of the cathode. Furthermore, the close spacing of the members or convolutions of the electrode l5 and the restriction of the activating material H to the interior of this electrode results in the capture of a high proportion of the positive ions outside the cathode by its negative external surface-which with connections and polarization such as described is naturally negative to the surrounding dischargediminishing the number that reach its interior to neutralize the negative space charge there. In this connection, it must be appreciated that the highly negative openwork members or convolutions exercise a strong attraction for the positive ions, so that any whose course takes them close to the members are drawn to the latter and captured, as well as those that spontaneously drive against the members. Hence an opening between the members that is sufliciently narrow is almost or virtually equivalent to no opening at all, so far as entrance of positive ions therethrough is concerned; while at the same time it permits comparatively free egress of electrons, which are repelled by the negative openwork members. Thus the closely spaced barrier members or convolutions l5 limit the positive ions entering the segregated region within, during discharge, and the current drawn from said region by the anode IE or A; though for starting, the anode I6 is rendered sufiiciently positive to draw discharge-initiating current despite the barrier, thus bringing about discharge to anode A, as already mentioned. In sum, the usual efiect of positive ions in augmenting cathode emission and total discharge in a cumulative manner is not produced; or, in other words, the auxiliary discharge does not show the usual negative voltagecurrent characteristic of a gaseous discharge: on the contrary, an increase in the voltage across the electrodes l5, I6 i required to increase the discharge. This remains true, however, only so long as the voltage across the electrodes l5, I6 is kept reasonably low, and the other conditions indicated are fulfilled.

The restrictive influences pointed out in the preceding paragraph are also effective as regards the discharge between the electrodes l5 and A; so that as long as the voltage across these electrodes is not too high in relation to the distance wire of 0.04 inch diameter.

between the electrodes, this over-all discharge also exhibits a positive voltage-current characteristic.

With an atmosphere in the envelope it comprising both starting gas and vaporizable working substance, such as argon and mercury, the discharge between the electrodes l5 and I6 tends to exhibit a double character, depending on the argon pressure and the cathode temperature, as well as on the voltage across the electrodes I 5, It. For example, with an argon pressure of mm., and a relatively low but emissive cathode temperature, the device for which illustrative dimensions are given below operates with a mercury discharge and glow at the electrodes l5, It When the voltage across them is about 11 volts; while with the same argon pressure and a higher cathode temperature, this device operates with an argon discharge and glow at the electrodes l5, it when the voltage across them is about 18-25 volts. Under both conditions of the discharge across the electrodes l5, l6 (argon glow and mercury glow), the device operates with a mercury glow for the positive column between the more widely spaced electrodes l5 and A, and shows a positive voltage-current characteristic under a voltage of to 50 volts across these electrodes. In general, the voltage for this main discharge between the more widely spaced electrodes l5 and A must be higher when the auxiliary discharge between the electrodes l5, l6 exhibits the argon glow than when it exhibits the mercury glow.

For the device to operate with positive voltagecurrent characteristics as above described, the electrode structure C-or, more especially, the electrode Ill-must be suitably proportioned, as already suggested. Here an important consideration is that the intervals between the members or wire turns of the electrode l5 should be so small that the members constitute an effective barrier against entrance of sui'licient positive ions into its interior to largely neutralize the negative space charge there, so that electrons could be emitted more freely to ionize the atmosphere outside the electrode coil l5, to augment the discharge cumulatively and produce a negative volt age-current characteristic. For the assistance of those desiring to use my invention, I will now give illustrative dimensions and other particulars of an actual device.

A device operating satisfactorily as above described and exhibiting positive voltage-current characteristics for both auxiliary and main discharges may be built with an envelope ID of hard glass (such as the nonex grade of Pyrex glass) of nominal 1 inch internal diameter; with an anode A formed by bending the end of a lead l2 of 0.04 inch nickel wire into a ring of about 1 inch diameter; with a gap of about 7% inches between this anode A and the adjacent end ofthe cathode coil IS; with a coil I5 of molybdenum wire of 0.02 inch diameter coiled 40 turns per inch to an internal diameter of 0.10 inch, and inch long; and with an electrode ring l6 having a diameter of about {'5 inch, formed of molybdenum Operating with an auxiliary voltage of 11 volts across the electrodes l5, it, this device shows a current of about 10 milliamperes for the auxiliary discharge as measured at 31, and a current of about 0.10 ampere for the main discharge, as measured at 38, under a voltage of volts across the main electrodes I5, A. However, it will be understood that'by suitably proportioning and arranging the electrodes l5, l6 and suitably spacing the electrode A from them, the voltages and currents for both discharges can be varied rather widelywithout sacrificing the positive current-voltage or. resistance characteristics of these discharges.

The electrode structure C shown in Fig. 3 differs from that in Figs. 1 and 2 in having for its auxiliary electrode an axially arranged wire or rod Ilia inside the cathode coil l5, which is in effect an extensionof the current lead I911; also, the current lead 22a from the inner endof the coil l extends outside this coil to the stem press 20 through which it is sealed; and this inner end of the coil I5 is closed by'a disc 39 of sheet metal (e. g., molybdenum) welded thereto, instead of by an insulator l8. Moreover, the circuit connections of the leads 2la and 22a are interchanged as compared with the leads 2| and 22 in Figs. 1 and 2. In other respects, this device and its construction, circuit connections, and operation may be essentially like those of the device shown in Figs. 1 and 2. Accordingly, variousparts and teatures in Fig. 3 are marked with the same reference numerals as the corresponding parts and features in Figs. 1 and 2, as a means of dispensing with repetitive description -a distinctive letter being added where such distinction appears necessary.

Fig. 4 illustrates a device with electrode A consisting of a sheet metal anode disc llb attached to the end of the support and current lead l2. Its.

electrode structure C comprises an openwork or grid-like electrode l5b, shown as a helical coil; without any special activation or provision for.

heating it, and an associated specially heated thermionic cathode lGb arranged inside the Screen-like electrode l5b and externally coated with activating material llb over a major portion of its surface. While the'electrode I Eb serves as anode for auxiliary discharge between it-and the cathode lBb, it may also function as cathode for the main discharge to the anode A, as explained hereinafter. The cathode liib' may consist of a. hollow metal cylinder with a heating coil unit 4!! inside it. Flanged refractory insulators l8b, l8b are fitted around the ends of the inner electrode l6b inside and overlapping the end convolutions of the coil l5b, to prevent electric discharge from passing freely through the ends of the coil. The

. Accordingly,

electrodes 15b and assign outer electrode lib into the atmosphere around it. The outer electrode lib acts as a starter anode in conjunction with the cathode Nibvery much as described above with reference to Figs. 1 and Z-providing excitation of the starting gas immediately adjacent the electrode l5b. Under this excitation, the device 'breaksdown to the anode A, and a positive column are discharge results between the electrodes l5b and A. As in the case of the devices shown in Figs. 1-3, the negative members or convolutions of the outer electrode lib constitute an effective barrier against entrance of suflicient positive ions formed in the gas around the electrode l5b' to neutralize the space charge around the cathode lib and being about freer emission of electrons, thus ultimately augmenting the discharge cumulatively. this Fig. 4 device exhibits positive voltage-current characteristics as regards both the auxiliary discharge between the electrodes l5b, llib'and the main discharge between the A. Both discharges start readily on normal operating voltages. However, the main discharge does not maintain itself without the auxiliary discharge; for.. asthe electrode I5!) is but a rather poor emitter of electrons at the temperatures which it'attains in operation, electron emission from the electrode I8?) is needed to produce the supply of ions on which the main discharge depends. The spacing of the members or convolutions of the outer electrode 1 l5b is determined by the same considerations as current supply connections shown in Fig. 4 resemble those of Figs. 2 and 3 in that. the elec trodes A and I5?) are connected across the main power circuit 30 and the electrodes l5b, lib across the auxiliary power circuit 32b; but instead of being derived from the main power circuit 30 as in Figs. 2' and 3, the auxiliary circuit is separately energized with A. C. by being connected across the secondary of a transformer 4| whose primary is connected across an A. C. power circuit 42, while the main circuit 30 is energized from-any suitable D. C. source 43. The D. C. resistance ofthe secondary of the transformer 4| will naturally be so low as to have no appreciable ballasting eflect. The proper polarization for this system of connections is such as to make the cathode llib negative to the electrode l5b when the anode A is positive relative to said electrode l5b. Various parts and features in Fig. 4 are marked with the same reference numerals as the corresponding parts and features in'Figs. 1 and 2', in order to dispense with repetitivedescriptiona distinctive letter being added where such distinction appears necessary.

In the operation of the device as shown in Fig. 4, electrons emitted from the cathode l6b issue between the members or convolutions of the inch between its adjacent convolutions.

in the case of the electrode IS. in Figs. 1 and 2; but for the benefit of those who may wish to use this form of device, I give illustrative dimensions and other particulars of an actual device.

A device operating satisfactorily-on voltages 20 to 50 volts across the main gap between electrodes A and I61) and 10-11 volts across the auxiliary gap between electrodes l5b and I6?) may bebuilt with an anode disc llb of sheet nickel about inch in diameter; with a. 7 inch gap between this anode and theadjacent ends of the electrodes l 5b, lGb; with an electrode shell lib of nickel about 1*; inch in diameter and 1 inch long activated by a coating of the usual mixture of barium and strontium oxides; and with an electrode coil l6b of molybdenum wire of 0.02 inch diameter coiled about 30 turns per'inch and 1% inch long, of about 0.100 inch internal diameter, spaced about 0.025 inch around the cathode l5 and having an interval of about 0.013

Operating with an auxiliary voltage of 11 volts across the electrodes l5, it, this device shows a current of about 10 milliamperes for the auxiliary discharge, as measured at 31; and with a voltage of 50 volts across its electrodes A and IE, it shows a current of about 0.10 ampere for the main discharge as measured at 38. Speaking more generally, it may be said that if the auxiliary current is less than 40 milliamperes and the interval or clearance within the coil l'fib around the electrode l 5?) is not over about 0.5 inch, these electrodes I517,

c llib show positive resistance characteristics and the device may be operated at proper auxiliary voltage without any ballast of any. kind in either of its discharge circuits.

is much steeper than with the Fig. 4 connections. While the electrode I6e serves as cathode for the main discharge, instead of the electrode I50 as in Fig. 1, the electrode 550 serves as a barrier against entry of positive ions into its interior, very much as in Fig. 4; for the connections are made such that both of the electrodes I50 and l6c are negative when the anode A is positive, though the barrier electrode 5c is less negative than the cathode I50. In Fig. 5, various parts and features are marked with the same reference numerals as the corresponding parts and features in Figs. 1-4 in order to dispense with repetitive description-a distinctive letter being added where such distinction seems necessary.

The device shown in Fig. 6 differs from those in Figs. 1-4 in having electrode structures C such as shown in Figs. 1 and 2 at both ends, instead of only at one end. This device is intended for A. C. operation, with each electrode structure C functioning alternately as cathode and anode on the alternate current cycles. the main A. C. power circuit 30d is connected across the two electrodes 15d, I5d. and the auxiliary or starting electrodes id, 5d are crossconnected t6 opposite sides of this circuit 30d by leads 32d, 32, each of which has a condenser 45 interposed therein, instead of a resistor as in Figs. 2 and 3. In Fig. 6, various parts and features are marked with the same reference numerals as are used for corresponding parts and features in Figs. 145 in order to dispense with repetitive descriptiona distinctive letter being added where such distinction appears necessary.

Fig. 7 shows a device with electrodes C like that in Fig. 4 at both ends, for A. C. operation as in Fig. 6, and with electrical connections similar to those shown in Fig. 5. As shown in Fig. 7, an autotransformer Me is connected across the main A. C. power circuit 30c, as well as the electrodes Hie, l6e; and the electrodes [56, l5e are connected by the leads 32c, 32a to taps on opposite sides of the mid-point of the transformer winding. Here the thermionic electrodes l6e. lBe serve as cathodes for the auxiliary discharge and as both cathodes and anodes, alternately, for the main discharge; while the barrier electrodes l5e, [5e also serve as auxiliary anodes. To dispense with repetitive description, the reference numerals used in Figs. 1-5 are applied to corresponding parts and features in Fig. 7, a distinctive letter being added where distinction appears necessary.

While I have described particular forms of construction and particular circuit connections, and even mentioned particular materials and dimensions, it is to be understood that these are given merely by Way of concrete illustration, and not as limiting or defining the invention. n the contrary, the device and its circuit connections can be varied in many ways that will readily-suggest themselves to those skilled in the art: the design, construction, materials, and arrangement of the electrode structures in the envelope or tube may be varied, as, for instance, the anode A and the coil electrode l may be of tantalum or various other metals; likewise, the gaseous and vaporizable substances for starting or for operation, and the pressures thereof, may be different; likewise, the system of connections may be altered. Moreover, the currents in the main, auxiliary, and heating circuits need not be of the same kinds-as is, indeed, exemplified in Fig. 4: above, where the 'main power circuit 30 is D. C., as well as the heating circuit, while the As shown in Fig.

auxiliary circuit 32 is A. C. The cathodes l5 and I5d and Nb, l5c, We may be activated with the usual mixture of barium and strontium oxides, or with any other suitable activating material, or may even be without activating coatings, if they are heated to temperatures that render them sufliciently emissive. In this latter case, the outer surfaces of the coil l5 or 15d would of course be rendered more or less unemissive by any suitable means.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrode structure for electric discharge in alow pressure atmosphere, said electrode structure comprising an internally activated but-externally unactivated openwork hollow cathode structure closed against free electric discharge to its interior, with means for passing current through said cathode to heat it to a temperature of internal emission, the openwork members being so closely spaced together that when maintained at negative potential relative to the surrounding discharge, while said cathode is functioning as such, they limit the positive ions entering the interior of the open work and the discharge current to be drawn from said interior by coacting anode means.

2. An electrode'structure for electric discharge in a low pressure atmosphere, said electrode structure comprising an internally activated but externally unactivated cathode coil, with means for passing current through said coil to heat it to a temperature of internal emission, and means for closing the ends of said coil against free electric discharge to its interior, the coil convolutions being so closely spaced together that when maintained at negative potential relative to the surrounding discharge, while said cathode is functioning as such, they limit the positive ions entering the interior of the coil and the discharge current to be drawn from said interior by coacting anode means. 1

3. An electrode structure for electric discharge in a low pressure atmosphere, said electrode structure comprising electron-emissive cathode means and barrier means enclosing the emissive cathode means and segregating and closing oif a region adjacent the latter against free electric discharge to said cathode means, while permitting free passage of the atmosphere of the device, said barrier means comprising screen members so closely spaced together that when maintained at negative potential relative to the surrounding discharge, while said cathode means is functioning as such, they limit the positive ions entering said region and the discharge current to be drawn from said region by coacting anode means, and screen members aforesaid having activated internal surfaces which serve as emissive cathode means.

4. An electrode structure for electric discharge in alow pressure atmosphere, said electrode structure comprising an externally unactivated openwork electrode structure with emissive cathode means in its interior closed off by said structure against free electric discharge to said cathode means, the openwork electrode members being so closely spaced together that when maintained at negative potential relative to the surrounding discharge, while said cathode means is functioning as such, they limit the positive ions entering the interior of the openwork and the discharge current to be drawn from said interior by coacting anode means, and openwork electrode members aforesaid being activated at their inner means.

5. A low pressure atmosphere electric discharge device comprising asealed envelope containing anode means and an opposed electrode structure, said electrode structure comprising electron-emissive cathode means for coacting with said anode means in an electric discharge and barrier means enclosing the emissive cathode means and segregating and closing of: a region adjacent the latter against free electric discharge to said cathode means, while permitting free passage of the atmosphere of the device,

astas sides, and thus serving as emissive cathode 

